1. Field of the Invention
The present inventive subject matter relates to solid state lighting devices.
2. Background of the Invention
Solid state light emitters including organic, inorganic, and polymer light emitting diodes (LEDs) may be utilized as an energy-efficient alternative to more traditional lighting systems. About ninety percent of the electricity consumed by an incandescent bulb is released as heat rather than light. In contrast to a typical 60-watt incandescent bulb that has an efficacy of about 13.3 to 14.2 lumens per watt, an LED light source can provide up to 200 lumens per watt.
Many modern lighting applications utilize high power solid state emitters to provide a desired level of brightness. High power solid state emitters generate heat that must be dissipated to prolong the life of the emitters. Generally, the lifetime of an LED is inversely related to the operating junction temperature thereof. Therefore, thermal management of the junction temperature is an important design consideration of a lighting device (or fixture) incorporating one or more LED's. For example, limiting the junction temperature of a particular LED manufactured by the assignee of the present application below 85° C. can result in an LED lifetime of approximately 50,000 hours. Operation of a such a solid state light source at a junction temperature of higher temperatures of 95° C., 105° C., 115° C., and 125° C. may result in life durations of 25,000 hours, 12,000 hours, 6,000 hours, and 3,000 hours, respectively. Many solid state lighting systems utilize a heat exchanger that dissipates heat into the ambient environment. Heat exchangers may be sized and shaped to maintain a specific solid state emitter junction temperature so as to obtain a desired life of the solid state emitters.
LEDs operate more efficiently when powered by a direct current (DC) voltage rather than an alternating current (AC) voltage. Solid state light emitting devices may typically be operated by control circuitry including an AC to DC converter because power is supplied to the device as AC voltage. The conversion circuitry (which may utilize a bulky transformer and one or more solid state electrical elements, such as diodes and one or more transistors) may be incorporated within the device thereby increasing fixture costs and space requirements. A more ready acceptance of LED lighting fixtures could be realized if size and costs could be reduced.
Heat exchangers are made of thermally conductive materials such as aluminum or an aluminum alloy. The heat flux that a heat exchanger can conduct depends on a variety of factors, such as the type and density of material, the surface area, the heat exchanger geometry, the thicknesses of the various surfaces, the convection coefficient of the ambient air flow, etc.
Further, a lighting device typically includes a reflector and a diffuser to direct light emitted from the solid state emitters. The reflector is made of a reflective material, such as aluminum or silvered plastic. The shape of the reflector in combination with the diffuser and LED array size, array configuration, and relative location of the array to other optical components produces a specific beam spread. The beam spread is the volume of space defined by the generally frusto-conical locus of points at which the intensity of the light is equal to 50% of the maximum lumen output. The beam spread determines the coverage of a single lighting unit as well as the spacing and quantity required when a plurality of such units are used for uniform illumination of a surface.